The present invention relates to a standards converter that converts a video signal from one television standard to another television standard, more particularly to a standards converter that also converts time codes.
Time codes are digital codes that are commonly inserted in the vertical blanking interval of a video signal to specify the hour, minute, second, and frame number of each video frame. When video signals are edited electronically by use of video tape recorders, time codes are an invaluable aid, as they enable edit points to be easily and precisely located.
In the NTSC television system employed in the United States, the frame numbers in the time codes take on thirty consecutive values in a cycle that repeats substantially once per second. In the PAL and SECAM television systems employed in Europe, the frame numbers take on twenty-five consecutive values in a cycle that repeats exactly once per second. Incidentally, NTSC is an acronym for National Television System Committee, PAL is an acronym for Phase Alternating Line, and SECAM is a French acronym for Sequential Couleur avec Memoire. Time-code formats have been standardized by the Society of Motion Picture and Television Engineers (SMPTE) and the European Broadcasting Union (EBU).
Television standards include the above-mentioned NTSC, PAL, and SECAM standards, variants of these standards, and various proposed high-definition television (HDTV) standards. It is known technology to convert a video signal from one television standard to another by digitizing the video signal, storing the digitized signal in a memory with a capacity of at least two fields (one frame), and reading the stored signal from the memory under appropriate address control. Specifically, the address control system is adapted to skip or repeat lines and fields as necessary for conversion between differing frame rates and different numbers of scanning lines per frame.
A problem is that conventional conversion apparatus for this type of conversion does not digitize or store the vertical blanking interval of the video signal, so the time codes are lost in the conversion process, making the converted video signal difficult to edit. Moreover, even if the vertical blanking interval were to be digitized and stored, the converted video signal would still present editing problems because, owing to the repeating or skipping of fields, the frame numbers in the converted time codes would not run in the normal consecutive sequence: frame numbers would be skipped or repeated. The resulting inconsistency of the frame numbers would make accurate identification of edit points troublesome and difficult.